Multiple Cracking of Unidirectional and Cross-PlyCeramic Matrix Composites

This paper examines the multiple cracking behavior of unidirectional and cross-ply ceramic matrix composites. For unidirectional composites, a model of concentric cylinders with finite crack spacing and debonding length is introduced. Stresses in the fiber and matrix are found and then applied to predict the composite moduli. Using an energy balance method, critical stresses for matrix cracking initiation are predicted. Effects of interfacial shear stress, debonding length and bonding energy on the critical stress are studied. All the three composite systems examined show that the critical stress for the completely debonded case is lower than that for the perfectly bonded case. For cross-ply composites, an extensive study has been made for the transverse cracking in 90° plies and the matrix cracking in 0° plies. One transverse cracking and four matrix cracking modes are studied, and closed-form solutions of the critical stresses are obtained. The results indicate that the case of combined matrix and transverse crackings with associated fiber/matrix interfacial sliding in the 0° plies gives the lowest critical stress for matrix cracking. The theoretical predictions are compared with experimental data of SiC/CAS cross-ply composites; both results demonstrated that an increase in the transverse ply thickness reduces the critical stress for matrix cracking in the longitudinal plies. The effects of fiber volume fraction and fiber modulus on the critical stress have been quantified. Thermal residual stresses are included in the analysis.     

Tensile and Fatigue Behavior of Silicon Carbide Fiber-Reinforced Aluminosilicate Glass

The onset of damage accumulation in ceramic-matrix composites occurs as matrix microcracking and fiber/matrix debonding. Tension tests were used to determine the stress and strain levels to first initiate microcracking in both unidirectional and cross-ply laminates of silicon carbide fiber-reinforced aluminosilicate glass. Tension–tension fatigue tests were then conducted at stress levels below and above the matrix cracking stress level. At stress levels below matrix microcracking, no loss in stiffness occurred. At stresses above matrix cracking, the elastic modulus of the unidirectional specimens exhibited a gradual decrease during the first 10 000 cycles, and then stabilized. However, the cross-ply material sustained most of the damage on the first loading cycle. It is shown that fatigue life can be related to nonlinear stress–strain behavior of the 0° plies, and that the cyclic strain limit was approximately 0.3%.     

Hysteresis loops of carbon fiber-reinforced ceramic-matrix composites with different fiber preforms

The hysteresis loops of C/SiC ceramic-matrix composites (CMCs) with different fiber preforms, i.e., unidirectional, cross-ply, 2D and 2.5D woven, 3D braided, and 3D needled at room temperature have been investigated. Based on fiber slipping mechanisms, the hysteresis loops models considering different interface slip cases have been developed. The effects of fiber volume fraction, matrix cracking density, interface shear stress, interface debonded energy, and fibers failure on hysteresis loops, hysteresis dissipated energy, hysteresis width, and hysteresis modulus have been analyzed. An effective coefficient of fiber volume fraction along the loading direction (ECFL) was introduced to describe fiber preforms. The hysteresis loops, hysteresis dissipated energy and hysteresis modulus of unidirectional, cross-ply, 2D and 2.5D woven, 3D braided and 3D needled C/SiC composites have been predicted.     

Preparation of SiC/SiC Composites by Hot Pressing, Using Tyranno-SA Fiber as Reinforcement

The development of advanced Tyranno SA SiC fiber with a near-stoichiometric composition and a well-crystallized microstructure has made it possible to prepare SiC/SiC composites even under harsh conditions. To assess the reinforcing effectiveness of Tyranno SA fiber at high temperature under pressure, unidirectional SiC/SiC composites were prepared by hot pressing, using pyrolytic carbon (PyC)-coated Tyranno SA fiber as a reinforcement and nanopowder SiC with sintering additives for matrix formation. The effects of sintering conditions on the microstructural evolution and mechanical properties of the composites were characterized. As the sintering temperature increased (from 1720° to 1780°C) and the sintering pressure increased (from 15 to 20 MPa), the density of the composites gradually increased. Simultaneously, the elastic modulus, the proportional limit stress, and the strength, under both bend and tensile tests, also improved. At lower temperature and/or pressure, long fiber pullout was a predominant fracture behavior, indicating relatively weak fiber/matrix bonding. However, at high temperature and/or pressure, short fiber pullout became a main fracture characteristic, indicating relatively strong fiber/matrix bonding. These phenomena were also confirmed by the characteristics of the hysteresis loops derived from the stress–strain curves produced by a tensile test with unloading–reloading cycles. In the present investigation, the reinforcement of Tyranno SA fiber is effective for providing noncatastrophic fracture behavior to composites.     

Interfacial Mechanical Characterization of Nicalon SiC Fiber/Alumina-Based Composites

Interfacial mechanical properties of both Nicalon SiC/aluminum borate and Nicalon SiC/aluminum phosphate with various fiber coatings and heat treatments were evaluated using a commercially-available indenter to induce fiber sliding during load cycling experiments. Varying degrees of sliding due to different coating materials were found. The interfacial characteristics including the shear, the residual axial fiber, and debond stresses were estimated by matching the experimental stress-displacement curves with curves predicted from an existing model. The elastic modulus and hardness of the interphase/interface in ceramic matrix composites were also evaluated. These results provided important insights into the ultimate mechanical performance of fiber-reinforced ceramic-matrix composites.     

Interfacial Mechanical Characterization of Nicalon SiC Fiber/Alumina-Based Composites

Interfacial mechanical properties of both Nicalon SiC/aluminum borate and Nicalon SiC/aluminum phosphate with various fiber coatings and heat treatments were evaluated using a commercially-available indenter to induce fiber sliding during load cycling experiments. Varying degrees of sliding due to different coating materials were found. The interfacial characteristics including the shear, the residual axial fiber, and debond stresses were estimated by matching the experimental stress-displacement curves with curves predicted from an existing model. The elastic modulus and hardness of the interphase/interface in ceramic matrix composites were also evaluated. These results provided important insights into the ultimate mechanical performance of fiber-reinforced ceramic-matrix composites.     

2D-SiC/SiC陶瓷基复合材料的拉伸本构模型研究

通过单向拉伸试验,研究了2D-SiC/SiC复合材料的应力-应变行为.结果表明,材料单向拉伸应力-应变曲线表现出明显的双线性特征,且线弹性段较长.通过试件断口照片,分析了2D-SiC/SiC复合材料单向拉伸破坏机理和损伤模式.基于对损伤过程的假设,建立了二维连续纤维增强陶瓷基复合材料的双线性本构模型,并将其应用于2D-SiC/SiC复合材料的应力-应变曲线模拟,模拟结果与试验值吻合很好.同时,分析计算表明,2D-SiC/SiC复合材料的单轴拉伸行为主要由纵向纤维柬决定,横向纤维对材料的整体模量和强度贡献很小.     

Failure of Crossply Ceramic-Matrix Composites

The fast-fracture and stress-rupture of a crossply ceramic-matrix composite with a matrix through-crack are examined numerically to assess the importance of fiber architecture and the associated stress concentrations at the 0/90 ply interface on failure. Fiber bridging in the cracked 0 ply is modeled using a line-spring bridging model that incorporates stochastic and time-dependent fiber fracture. A finite-element model is used to determine the stresses throughout the crossply in the presence of the bridged crack. For both SiC/SiC and a typical oxide/oxide, the fast-fracture simulations show that as global failure is approached, a significant fraction of fibers near the 0/90 interface are broken, greatly reducing the stress concentration. For fibers with low Weibull moduli ( m < 10), the tensile strength is thus nearly identical to that of a unidirectional composite scaled by the appropriate fiber volume fraction, while for fibers with larger Weibull moduli ( m ≥ 10), there are modest (10−17%) reductions in tensile strength. Stress-rupture simulations show that initially high stress concentrations are relieved as fibers fail with evolving time near the 0/90 interface and shed load away from the interface. For a wide range of fiber properties, efficient load redistribution occurs such that the crossply rupture lifetime is generally within an order of magnitude of the unidirectional lifetime, when the applied stress is normalized by the relevant fast-fracture strength. Overall, stress concentrations at the 0/90 interface are largely relieved with increasing load or time due to the nonlinear bridging response and preferential fiber failure near the interface, resulting in crossplies that respond very similarly to unidirectional composites.     

Ultrasonic Evaluation of Initiation and Development of Oxidation Damage in Ceramic-Matrix Composites

In this paper we report on the development of a method for ultrasonic nondestructive characterization of oxidation damage in ceramic-matrix composites. The method is based on ultrasonic measurement of elastic moduli of the composite, which are then used to determine the elastic moduli of the fiber-matrix interphase. Thus the interphasial damage may be estimated quantitatively. As a model system we used, to demonstrate applicability of the method, a unidirectional SiC-fiber-reinforced reaction-bonded silicon nitride matrix composite (SiC/RBSN). The composite samples were oxidized in flowing oxygen for 0.1, 1, 10, and 100 h at 600°, 900°, 1200°, and 1400°C. The ultrasonic phase velocity in the composite was measured at room temperature before and after oxidation; the data were then used to find the composite moduli, which quantify the induced damage. Significant changes in ultrasonic velocities and composite moduli were found as a result of oxidation. Fiber-matrix interphasial moduli were determined by multiphase micromechanical analysis. We found that oxidation of the carbon interphasial layer is the dominant mechanism in decreasing the elastic moduli of the composite. The critical exposure time for transition from the nondamaged to the damaged state at different oxidation temperatures has been determined.     

Multiple Cracking and Tensile Behavior for an Orthogonal 3-D Woven Si-Ti-C-O Fiber/Si-Ti-C-O Matrix Composite

This paper presents experimental results for the multiple microcracking and tensile behavior of an orthogonal 3-D woven Si-Ti-C-O fiber (Tyranno™ Lox-M)/Si-Ti-C-O matrix composite with a nanoscale carbon fiber/matrix interphase and processed using a polymer impregnation and pyrolysis route. Based on microscopic observations and unidirectional tensile tests, it is revealed that the inelastic tensile stress/strain behavior is governed by matrix cracking in transverse (90°) fiber bundles between 65 and 180 MPa, matrix cracking in longitudinal (0°) fiber bundles between 180 and 300 MPa, and fiber fragmentation above 300 MPa. A methodology for estimation of unidirectional tensile behavior in orthogonal 3-D composites has been established by the use and modification of existing theory. A good correlation was obtained between the predicted and measured composite strain using this procedure.     

In-Plane Cracking Behavior and Ultimate Strength for 2D Woven and Braided Melt-Infiltrated SiC/SiC Composites Tensile Loaded in Off-Axis Fiber Directions

The tensile mechanical properties of ceramic matrix composites (CMC) in directions off the primary axes of the reinforcing fibers are important for the architectural design of CMC components that are subjected to multiaxial stress states. In this study, two-dimensional (2D)-woven melt-infiltrated (MI) SiC/SiC composite panels with balanced fiber content in the 0° and 90° directions were tensile loaded in-plane in the 0° direction and at 45° to this direction. In addition, a 2D triaxially braided MI SiC/SiC composite panel with a higher fiber content in the ±67° bias directions compared with the axial direction was tensile loaded perpendicular to the axial direction tows (i.e., 23° from the bias fibers). Stress–strain behavior, acoustic emission, and optical microscopy were used to quantify stress-dependent matrix cracking and ultimate strength in the panels. It was observed that both off-axis-loaded panels displayed higher composite onset stresses for through-thickness matrix cracking than the 2D-woven 0/90 panels loaded in the primary 0° direction. These improvements for off-axis cracking strength can in part be attributed to higher effective fiber fractions in the loading direction, which in turn reduces internal stresses on weak regions in the architecture, e.g., minicomposite tows oriented normal to the loading direction and/or critical flaws in the matrix for a given composite stress. Both off-axis-oriented panels also showed relatively good ultimate tensile strength when compared with other off-axis-oriented composites in the literature, both on an absolute strength basis as well as when normalized by the average fiber strength within the composites. Initial implications are discussed for constituent and architecture design to improve the directional cracking of SiC/SiC CMC components with MI matrices.     

Transverse Thermal Conductivity of Thin C/SiC Composites Fabricated by Slurry Infiltration and Pyrolysis

Thin C/SiC composites were fabricated by infiltrating a woven carbon fiber fabric with a slurry of SiC powder and polymer precursor for SiC, followed by heat treatment for pyrolysis. The effects of heat treatment parameters on the crystallization of the polymer-derived SiC, the composite microstructure, and the transverse thermal properties were assessed. Whereas composites heat-treated at 1000°C were crack-free and nearly fully dense, composites that were subjected to further multiple reinfiltration and heat treatment cycles at 1600°C developed porosity and cracking. However, the transverse thermal conductivity was increased significantly by the higher-temperature heat treatment, to values higher than that of a composite with a chemical-vapor-infiltration SiC matrix and the same fiber reinforcement.     

碳化硅晶须增韧陶瓷基复合材料的研究进展

碳化硅晶须具有高强度、高弹性模量等优点,是一种应用广泛的补强材料。本文结合国内外研究现状着重介绍了碳化硅晶须的性质、增韧机理以及在增强陶瓷基复合材料中的研究进展,探讨了新的生产工艺和研究方向,并对碳化硅晶须的应用前景做了展望。     

Micromechanical analysis of long fiber‐reinforced composites with nanoparticle incorporation into the interphase region

The influence of the distribution type, Young's modulus, and volume fraction of the nanoparticles within the interphase region on the mechanical behavior of long fiber‐reinforced composites with epoxy resin matrix under transverse tensile loading is investigated in this article. An infinite material containing unidirectional long fiber and periodic distribution of elastic, spherical nanoparticles was modeled using a unit cell approach. A stiffness degradation technique has been used to simulate the damage and crack progress of the matrix subjected to mechanical loading. A series of computational experiments performed to study the influence of the nanoparticle indicate that the mechanical properties, nanoparticle‐fiber distance, and volume fraction of nanoparticle have a significant effect on both the stiffness and strength properties of these composite materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41573.     

Flexural modulus of SiC/SiC composites sintered by microwave and conventional heating

To deeply study the variation mechanisms of mechanical properties, flexural modulus of SiC fibers reinforced SiC matrix (SiC/SiC) composites prepared by conventional and microwave heating at 800?°C–1100?°C was discussed in detail. The elastic modulus of fibers and matrix, interface bonding strength and porosity of SiC/SiC composites were considered together to analyze the changing tendencies and differences in their flexural modulus. The elastic modulus of fiber and matrix was determined by nanoindentation technique and interface characteristics applying fiber push-out test. The porosity and microstructure examinations were characterized by mercury intrusion method, X-ray Diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Moreover, two conflicts between the changing trends of elastic modulus and chemical compositions of composite components were focused and explained. Results indicate that three factors played different roles in the flexural modulus of SiC/SiC composites and residual tensile stress in composite components led to the conflicts between their elastic modulus and chemical compositions.     

Characterization and application of a novel low viscosity polysilazane for the manufacture of C- and SiC-fiber reinforced SiCN ceramic matrix composites by PIP process

Four unidirectional fiber reinforced SiCN ceramic matrix composites were manufactured by means of polymer infiltration and pyrolysis. Two carbon fibers (T800H and Granoc XN90) as well as two silicon carbide fibers (Tyranno ZMI and SA3) without fiber coating were chosen. As matrix precursor, a poly(methylvinyl)silazane was investigated and utilized. The composites with the SA3 and the XN90 fiber had the highest tensile strengths of 478 and 288 MPa, respectively. It is considered that these high modulus fibers with the low modulus SiCN matrix create weak matrix composites. After exposure to air (T = 1200 °C, 10 h), a significant decrease of the mechanical properties was found, caused by the burnout of carbon fibers and the oxidation through open pores stemming from the PIP process and SiCN/SiCN interfaces in case of the SiC fiber based composites.     

Effect of thermal cycling on carbon fiber‐reinforced PPS composites

Calorimetry, coefficient of thermal expansion (CTE), and tensile modulus were recorded to investigate the effect of thermal cycling on polyphenylene sulfides (PPS) carbon fiber composites. Thermal cycling at higher temperatures increased the degree of crystallinity of PPS, as indicated by increasing heat of melting. CTE measurements during thermal cycling were used to study the anisotropy of the composites in directions parallel and transverse to the fiber orientation. It was noted that increasing crystallinity enhanced the tensile modulus of unidirectional composites, while reducing the tensile modulus of quasi‐isotropic composites. The latter reduction may be due to internal damage or interlaminar slippage associated with the residual thermal stresses caused by thermal mismatch between multiply oriented plies. POLYM. COMPOS., 26:713–716, 2005. © 2005 Society of Plastics Engineers     

Methodology for Relating the Tensile Constitutive Behavior of Ceramic-Matrix Composites to Constituent Properties

A methodology for the straightforward and consistent evaluation of the constituent properties of ceramic-matrix composites (CMCs) is summarized, based on analyses from the literature. The results provide a constitutive law capable of simulating the stresshain behavior of these materials. The approach is illustrated using data for two CMCs: SiC/CAS and SiC/SiC. The constituent properties are also used as input to mechanics procedures that characterize stress redistribution and predict the effect of strain concentrations on macroscopic performance.     

Oxidation of BN/Nicalon Fiber Interfaces in Ceramic-Matrix Composites and Its Effect on Fiber Strength

Microstructural changes at the interface were analyzed in two Nicalon-fiber ceramic-matrix composites with a dual BN/SiC coating on the fibers after thermal exposure at different temperatures (in the range 800°-1400°C) and in different environments (air and argon). The outer SiC coating acted as a barrier to oxygen, which penetrated into the composite via pipeline diffusion along the BN/fiber interfaces. Oxygen penetration led to the formation of an SiO₂ layer by oxidation of the fiber surfaces. The in situ fiber strength at different temperatures, as determined from the radius of the mirror region on the fiber fracture surface, indicated that this SiO₂ layer severely degraded the fiber strength. Oxidation was highly dependent on the nature of the BN/fiber interface. The presence of a thin carbon-rich interlayer, which burned out rapidly at high temperature, favored the entry of oxygen and accelerated oxidation of the fibers.     

Elastic Response and Effect of Transverse Cracking in Woven Fabric Brittle Matrix Composites

This paper examines the linear elastic tensile and fracture behavior of biaxial plain weave SiC/SiC ceramic woven fabric composites. Iso-phase mode and random-phase mode have been adopted to simulate multilayer stacking and to predict the initial linear elastic constants. It has been found that both modes predict very close results. Porosities in the composite affect the stiffness significantly, while fiber undulation shows only minimal effect. The nonlinear stress-strain relation of the composite is due to transverse cracks, which initiate mainly from interyarn pores. In the second part of this paper, two methods, classical fracture mechanics and energy balance approach, have been used to examine the crack initiation and growth. A finite element method and a modified shear-lag method have been developed to evaluate the stress distribution in the yarn with transverse cracks. The composite stiffness reduction due to transverse cracking has been obtained by both the finite element and shear-lag methods. Strain energy release rates of the growth of transverse cracks have been studied by the crack-closure procedure, using finite element methods. Effects of the yarn size and crack position on the strain energy release rate have been quantified. It is concluded that thinner yarns lead to higher critical strains for transverse cracking.